The invention relates to a measuring device and particularly to a photoelectric length or angle measuring device for measuring the relative position of two objects.
Position measuring devices are used, for example, in processing machines for measuring the relative position of a tool and a workpiece. Typically, a scanning unit connected to the tool scans a graduation connected to the workpiece. The signal obtained as a result of scanning is used to determine the relative position of the tool to the workpiece. As an additional example, position measuring devices are used in coordinate measuring machines for determining the position and/or dimensions of test objects.
The dissertation "Three-Lattice Interval Indicators" by Jorg Willhelm, 1978, describes, in Section 2.4.1, an incident light measuring device in which the graduation of a graduation support is scanned by a scanning unit. The scanning unit comprises a light source, a condenser, a scanning plate with a scanning graduation, an objective lens, and a photoelement. The light beams of the light source are directed in parallel from the condenser and traverse the scanning graduation of the scanning plate. The light beams are reflected from the graduation plane of the graduation support, through the scanning graduation and the objective lens, to the photoelement. Two displaceable objects are connected with the graduation support and the scanning unit, respectively. In an evaluating device, measuring values of the relative position of the two displaceable objects are determined from periodic scanning signals produced by the photoelement during the measuring movement. The evaluating device includes a trigger unit and a meter.
Section 4.7 of the dissertation states that flatness errors of the graduation plane of the graduation support, as well as guide errors of the graduation support or the scanning unit, can lead to measurement errors. These measuring errors are especially prominent in interferometric incident light measuring devices because the scanning distances between the graduation plane of the graduation support and the scanning graduation plane of the scanning plate are greater.